A DNA string is not that complex if you reduce its appearance to the four nitrogen bases
Adenin, Cytosin, Guanin and Thymin, short A, C, G and T. These bases are complementary
so that A comes together with T and C with G. So there arent't that much combinations
to get a correct DNA string.
Furthermore the two strings that one DNA double string is made of are anti-parallel, so that
you can "read" one string from left to right and the other one from right to left.
A DNA string also is made of a rest of phosphore and ribose or desoxyribose. The phosphore rest
is located at the "5'-end" and the ribose at the "3'-end". Normally you read a DNA string from
the 5'-end to the 3'-end.

On the next pages I will show some examples what is a incorrect DNA string and what is a correct one.